By virtue of the gradually improved image quality and the more simplified PC interfaces, digital cameras have become very popular, which are able to shoot digitized images and thereby facilitate the transmission of images through the internet and the printing of images by a color printer. Nowadays, digital camera has almost become a necessity in our daily lives. As the digital techniques become more and more widespread, it is not difficult to utilize various kinds of software to modify/edit digital pictures, which makes the alteration of the information recorded in digital pictures convenient. But the authenticity and integrity of the digital pictures, such as pictures of insurances, contracts, bills, notes, etc., are especially important when those pictures are to be used as electronic evidences. Therefore, the matter of identifying the authenticity and integrity of digital pictures comes forth, which is solved in the present invention by a built-in picture authenticity-protection device.
Digital watermarking techniques can effectively prevent digital images from falsification, which can be divided into two different types on the basis of the object. The first type is robust watermarks, which are used to protect copyrights and provide legal basis when copyright disputes happen. The second type is fragile watermarks (or half fragile watermarks), which are used to detect if multimedia products have been maliciously altered or damaged. With regard to robust watermarks, they must be robust enough so that when multimedia products are attacked and altered, they can still be extracted correctly to prove the copyright of the author, whereas fragile watermarks are mainly used to prove the authenticity of multimedia products. At Present, multimedia pictures cannot be used as evidences before courts, simply because multimedia pictures can be easily modified by various kinds of software (e.g. Photoshop) without being detected and even a tiny change of medical images may result in wrong judgments of the states of illnesses by doctors. Thus, the identification of the authenticity of multimedia products is much more important than the identification of copyright at these occasions.
Please refer to the literatures listed below for the related prior arts about digital watermarks and digital cameras.    [1] G L. Friedinan “The trustworthy digital camera restoring credibility to the photographic image” IEEE Trans, Consumer Electron, Vol. 39, PP. 905-910, November 1993    [2] S. Walton, Information authentication for a slippery new age [J], Dr. Dobbs Journal, 1995, 20(4): 18-26    [3] CN1184294A “System for embedding authentication information into an image and an image alteration detecting system”, application number: 97121196.9, application date: Oct. 27, 1997    [4] Wong P W. A public key watermark for image verification and authentication [A]. In: Proc of the IEEE International Conference on Image Processing (ICIP'98) [C], Chicago, Ill., USA, 1998, 1:455459
In Literature [1], Friedman puts forward a method based on digital signatures to identify the authenticity of pictures taken by digital cameras. This method utilizes public key system and generates digital signatures of images according to the secret key of the camera. The digital signatures are saved in other files and are distributed together with images. In this way, users may identify the authenticity of the image via the combination of public key, the image file and the digital signature file. But it requires more storage space and it is inconvenient to manage the digital signature information. Besides, the method cannot detect distortions.
Literature [2] provides a method to calculate the eigenvalue of the image using checksum. In this way the authenticity of the image is not so guaranteed when compared with hash algorithm because by using checksum, a same value might be obtained from different images.
The Chinese patent CN1184294A [3] divides a image into two regions and the hash value is calculated from one region of the image, then the hash value obtained is embedded in the other region of the image by method of pixel-block encoding and finally, the two regions are combined together by a region-combination unit. A big defect of this method is that the region in which the image has been altered cannot be located.
Literature [4] provides a public key watermark method for image authentication. The watermark algorithm of the digital image in this method utilizes image blocks to locate the altered region. But the speed of taking pictures will be slowed down if this method of processing images by blocks is used in digital cameras. Therefore, the practical applicability of using this method in digital cameras is not ideal.
Also, there has been a great variety of sophisticated algorithms for calculating the eigenvalue of images in the prior art. For instance, the SHA-1 algorithm, the description and the C language source code of it can be found in a most authoritative document “rfcs 3174” on the internet (http://www.fags.org/rfcs/rfc3174. html).